Obesity and, especially, rapid weight loss are well-recognized risk factors for gallstone formation. Cholesterol supersaturation of bile has been well described in obese subjects undergoing rapid weight loss. The mechanism that results in bile supersaturation has not been elucidated, but one pathogenetic factor in cholelithiasis is inhibition of bile acid synthesis. Our hypothesis is that cholesterol supersaturation of bile occurs due to a relative decrease in bile acid biosynthesis which, it turn, is due to an increase in hepatic insulin sensitivity. The specific aim of the project is to determine whether inhibition of bile acid synthesis occurring during weight loss correlates with hepatic insulin sensitivity. In the intact human, bile acid synthesis will be measured using isotope dilution-mass spectrometry. In order to interpret changes in bile acid synthesis, certain key regulatory pathways will be simultaneously quantitated: biliary lipid composition, cholesterol synthesis and esterification by peripheral blood mononuclear cells. Hepatic insulin sensitivity will be determined by the euglycemic clamp insulin infusion method with simultaneous infusion of triated glucose. The analytical methods employed will include: isotope dilution - mass spectrometry; gas liquid scintillation spectrometry. Preliminary data demonstrates a consistent reduction in bile acid synthesis (approximately 60%) in all subjects after eight weeks on a liquid formula diet (15% decrease in body weight). We anticipate that the inhibition of bile acid synthesis that occurs with weight loss will correlate directly to an increase in hepatic insulin sensitivity. Those subjects experiencing the greatest increase in insulin sensitivity will exhibit the greatest inhibition of bile acid synthesis.